What Is an Open Invoice? How to Track and Collect Outstanding Invoices

Your accounting software shows 47 open invoices. Some are a week old. Some are pushing 90 days. You know the big ones by heart, but that $800 invoice from January? You forgot about it until right now.

An open invoice is simply an invoice that hasn't been paid yet. That sounds straightforward, but the term covers everything from an invoice sent yesterday to one that's been sitting unpaid for four months. The difference between those two situations is enormous, and most small businesses don't have a system for telling them apart.

Here's what open invoices actually mean for your business, how they differ from outstanding and overdue invoices, and what you can do to collect them before they turn into bad debt.

What is an open invoice?

An open invoice is any invoice that has been issued to a customer but has not yet been fully paid. It sits on your books as accounts receivable: money that's owed to you but hasn't arrived.

Open invoices include invoices that are still within their payment terms (the customer has time left to pay) and invoices that are past due. The term doesn't tell you whether there's a problem. It just tells you that cash hasn't come in yet.

For most B2B companies, open invoices make up a significant share of working capital. U.S. small businesses collectively hold $825 billion in unpaid invoices. That's money tied up in other people's bank accounts instead of yours.

Open vs outstanding vs overdue invoices

These three terms get used interchangeably in everyday conversation, but they describe different stages of the same invoice lifecycle.

Open invoice: Any unpaid invoice, regardless of whether it's within payment terms or past due. This is the broadest category.

Outstanding invoice: Functionally the same as an open invoice. In practice, "outstanding" often carries a slightly more urgent connotation. When someone says "we have outstanding invoices," they usually mean invoices that are approaching or past their due date.

Overdue invoice: An invoice that has passed its payment deadline. If your terms are Net 30 and it's day 31, that invoice is overdue. This is the narrowest category and the one that requires action.

The distinction matters for your accounts receivable process. An open invoice that's 5 days old and within terms needs no follow-up. An outstanding invoice at day 28 needs monitoring. An overdue invoice at day 45 needs a phone call. Treating them all the same leads to either over-managing (annoying customers who plan to pay on time) or under-managing (ignoring invoices that are sliding toward bad debt).

How to track open invoices

The single most useful tool for tracking open invoices is an aging report. It groups your open invoices by how long they've been unpaid: current (within terms), 1-30 days overdue, 31-60 days, 61-90 days, and 90+ days.

Most accounting software generates aging reports automatically. If you're still tracking invoices in a spreadsheet, build one. The insight is worth the effort: 41% of finance leaders say a lack of visibility over accounts receivable is their biggest cash flow challenge.

An aging report does three things:

Shows you where the money is. If $200,000 is in the "current" bucket and $15,000 is in the 90+ bucket, you have a manageable situation. If those numbers are reversed, you have an emergency.

Reveals patterns. The same three customers showing up in the 60+ bucket every quarter? That's not bad luck. That's a credit risk pattern you need to address with tighter terms or upfront payment requirements.

Prioritizes your follow-up. You can't chase every invoice with equal intensity. The aging report tells you which ones need attention right now and which ones can wait.

Review your aging report weekly. Not monthly, not quarterly. Weekly. The businesses that maintain low bad debt rates do it by catching problems at 15 days, not 90.

How to collect outstanding invoices

The approach that works isn't complicated, but it needs to be consistent. The biggest reason invoices age into bad debt isn't that customers refuse to pay. It's that nobody follows up.

Days 1-7 past due: a reminder. Send a payment reminder within the first week. Keep it factual, not apologetic. Include the invoice number, amount, and a direct payment link. Assume it was an oversight.

Days 8-15: a second reminder with a call. If the email didn't work, pick up the phone. A conversation reveals what email can't: disputes, cash flow problems, lost invoices, approval delays. Phone-based follow-up has a significantly higher response rate than email alone.

Days 16-30: escalate internally. At this point, the invoice is a problem. Involve a manager or the business owner. Send a formal reminder that references your payment terms and mentions that continued non-payment will result in further action.

Days 31-60: formal demand. Reference the possibility of statutory interest, recovery costs, and third-party collection. In the EU, the Late Payment Directive entitles you to charge interest and a minimum EUR 40 recovery fee on overdue B2B invoices. Even if you don't enforce it, referencing it signals seriousness.

Days 60+: collection or legal action. If you've followed every step and still haven't been paid, it's time for external help. Waiting longer only reduces your odds of collecting.

The businesses that collect the most consistently aren't the ones with the harshest tactics. They're the ones that follow up on every invoice, every time, without exceptions.

When open invoices become a cash flow problem

A handful of open invoices is normal. Every B2B business has them. The problem starts when the volume or aging of those invoices outpaces your ability to manage them.

Over 55% of all B2B invoiced sales in the U.S. are currently overdue. Nearly half of businesses report invoices overdue by more than 30 days. And 30% of small businesses say outstanding invoices create cash flow problems that threaten their ability to stay in business.

The compounding effect is what kills. One overdue invoice is an inconvenience. Twenty overdue invoices mean you can't make payroll, can't invest in growth, and can't take on new work because you're still financing the old work. In the EU, 25% of all bankruptcies are directly caused by late payments from customers.

Watch these warning signs:

• Your aging report shows a growing 60+ day bucket
• The same customers keep appearing in older aging categories
• You're extending credit to new clients without checking their payment history
• Your DSO (days sales outstanding) is climbing quarter over quarter
• You're using new revenue to cover gaps left by uncollected old revenue

If any of those describe your situation, the fix isn't just "send more emails." It's building a structured follow-up process that catches every open invoice before it ages past the point of no return.

Keeping open invoices from becoming bad debt

Open invoices are a normal part of B2B business. The difference between companies with 2% bad debt and companies with 8% bad debt isn't their customer base. It's their follow-up process.

The best-performing AR operations share a few traits: they invoice immediately after delivery, follow up within 7 days of the due date, use phone calls (not just email), and have a clear escalation path for every aging stage.

Dunwise automates that entire process with an AI voice agent that contacts every overdue customer on schedule. The tone adjusts to the aging stage automatically: conversational at 7 days, professional at 30, direct at 60. Every call outcome gets logged: promise-to-pay dates, dispute details, payment confirmations. Your open invoices get follow-up without taking your time.

The invoices that turn into bad debt are almost always the ones nobody followed up on. The sooner you act on an open invoice, the more likely you are to collect it.

Key takeaways

• An open invoice is any unpaid invoice, whether within terms or overdue. "Outstanding" and "overdue" are more specific stages of the same lifecycle.
• Track open invoices with a weekly aging report. Catch problems at 15 days, not 90.
• Follow up consistently: reminder at day 7, call at day 15, formal demand at day 30, escalation at day 60.
• Over half of B2B invoices in the U.S. are overdue, and 30% of SMBs say outstanding invoices threaten their business.
• The difference between low and high bad debt rates is follow-up consistency, not customer quality.